Stratigraphic studies in the Aktau Mountains bordering the Dzhungarian Alatau Range in southeastern Kazakhstan have included mapping of Tertiary lithostratigraphic units, documentation of fossiliferous deposits, correlation of sections, etc. These investigations have led in turn to revised interpretation of the Tertiary geology of the area. The Tertiary sequence in the Aktau Mountains is represented by three lithostratigraphic units (in ascending order): (1) the middle Eocene Akbulak Formation; (2) the Oligocene Aktau Formation with a lower member including white quartz sands that contain fossil mammals, and an upper member including red-colored clays and sandstones, brick red clays, an anhydrite and gypsum clayey horizon, and bright brown-red clays; and (3) the upper Oligocene-Miocene Chul'adyr Formation with a lower member of greenish and yellowish conglomerates and gritstones, a middle member including grayish and yellowish sands and gritstones, and an upper member including brown and red clays and carbonate- and anhydrite-rich clays. The Aktau and Chul”adyr Formations represent separate cycles of sedimentation. Mammalian biostratigraphy and biochronology of the three vertebrate faunas in Aktau Mountains are reviewed. The mammalian fauna from white sands of the lower Aktau Formation is small but includes Ardynia and is thought to be early Oligocene in age. The mammalian fauna from conglomerates and gritstones of the lower member of the Chul”adyr Formation is also small but includes Paraceratherium and is thought to be late Oligocene in age. The mammalian fauna from sands of the middle member of the Chul'adyr Formation is extensive, with micro- and macrofauna attributed to Neogene mammal zones MN4 to MN 6, indicating a latest early Miocene to earliest middle Miocene age (Orleanian-Astaracian). Most genera of middle Chul”adyr mammals are known from the middle Miocene Shanwang faunas of China and from the Castelnau-d”Arbieu faunal assemblage (MN4-MN6) of southwestern France.